Antiskid control system



O United States Patent [111 3,545,819

[72] Inventors William Charles Gaflney 3.245.727 4/1966 Anderson et303/2l M; 3,398,995 8/1968 Martin 303/21 Ervla G. Romero, Seattle,Washington 3,441,320 4/l969 Flo 303/2! [2|] Appl. No. "6,724 3,450,4446/l969 Ballard 303/21 [22] Filed April 16, I969 3,494,67! 2/ i970 Slavinet al. 303/21 [45] Patented Dec. 8, 1970 Prime E a ry summer-Duane A.Reger i m W Attorneys -G|enn Orlob, Kenneth W. Thomas and Consam't m radO. Gardner a corporation of Delaware 5 mxm omo SYSTEM ABSTRACT: Allantiskid control system which includes cm 5 Day M means for preventinginteraction between the system and the I natural oscillation of thelanding gear. A frequency clamping i PL 303/21 ,circuit arrangementincludes a one shot multivibrator which 244/ 1 303/20 controls a switchin the path of the valve dump voltage. The U" multivibrator is set bythe removal of an antiskid control [50] Field olSearsh 303/20, 21; and.he a nae f he m i is f a time period 244/ I 1 l sufficient to preventthe application of a second dump voltage antiskid control signal to thevalve within the natural resonant [56] cm frequency period of thelanding gear. This prevents the an- UNITED STATES PATENTS tisltid systemfrom sustaining an oscillation of the gear and sta- 3.245.213 4/1966Thompson et al. 303/21 bilizes the skid control system.

20 r a PRESSURE 3 4 souRcE lvh'EEL SKID PEEP nsrscror i i VALVE wave armDRIV I? l0 /2 DUI, f

awe/ m 1 a D 6. ONE SHOT swrcu MV PATENTEU DEC 8 I976 SHEET 1 OF 2PRESSURE SOURC E VAL VE DUMP VOL 7/165 SKID QPEE D WHEEL OIYE 57/07 D.C. 26 SWITCH VAL VE DUMP VOL TAGE MODUL A T/ON VOL TAfiE TIME OUTPUT 0FV, l 0 6 MW 4; wMF 0 mm T N66 flu W M mwm W A v mm M v, E M 5 0 TEIII1|l|7 m M t 5/ In C y IMIIIIINW m w |1 w n la ATTORNEY g ANTISKIDCONTROL SYSTEM This invention relates to brake control systems forvehicles, and more particularly to antiskid control type brake controlsystems.

It is desirable to prevent skidding of a braked wheel and even moredesirable to further sense incipient skids and provide control of awheel being braked before the overtorqued condition is reached. Antiskid control systems have been developed which .detect an actual skidand provide a control signal of the type termed a dump voltage which isapplied to the servopilot .valve thereby bypassing fluid under pressurearound the brake cylinder. The servopilot valve (a flappernozzlevalve)provides bypassing of fluid in proportion to the amplitude of theapplied control signal. A dump voltage is provided to-the valve when anactual skid condition occurs and is a maximum amplitude applied controlsignal which causes maximum relaxation ofbraking effort necessary whenthe wheel has decelerated into an actual skid and it is necessary tobring the wheel back up to aspeed where braking action can again becomeeffective. A further class'of present state of the art antiskid systemsnow also detects incipient skid conditions and provides a control signalwhich is applied to the servo valve which is of less amplitude that thanthe aforementioned dump voltage" type control signalof maximumamplituderequired when an actual skid condition is detected. These state of theart systems provide a control signal to the servo valve whichismodulated in accordance with the information detected during incipientskid conditions and which causes as a consequence the modulation ofbrake pressure in accordance with the preskid information detected andprocessed.

Improvement in antiskid control systems is critically dependent uponresponse of the system to incipient skid control information detected.The great difficulty encountered in achieving thisobjective'bymaintaining high sensitivity and frequency response has beenthe occurrence of instability in proposed systems attempting to achievethese critical objectives. The instability of proposed antiskid controlsystems has been caused by the introduction into the system of geardeflection developed information along with the desired skid and preskidinformatiomThe wheel support member or landing gear of an aircraft isnot completely rigid and braking force when applied causes deflection ofthe landing gear rearward, which results in wheelspeedde'celeratidmcausing the system to react to this information in thesamemanner as if the wheel deceleration were due to an incipient skid andresults in a control signal being applied to the valve releasingthe'brakes. This brake by the antiskid control system and'anotherrearward deflection of the gear. Subsequent release of brakes causes increased amplitude forward deflection of the landing gear. The amplitudesof landing gear oscillation may be caused to increase in this mannertoward the strength limits of the structure. It is most desirable thatan antiskid control system respond to preskid information by generatinga control signal based on this information for-application to the valveto release brake pressure as required, and it is also important toprovide a dump voltage to the valve for, maximum brake release when anactual skid-occurs; however, landing gear deflection informationgenerated when gear oscillation occurs causing as in a skid conditionthe occurrence of a dump volt-- age must not be permitted to repeatitself, thereby contributing to increased gear oscillation at thenatural resonant frequency of vibration of the gear structure.

It is therefore an object of this invention to provide for reducing thedeleterious effectsof landing gear vibrations on It is still anotherobject of this invention to provide means in an antiskid control systemwhich permits the system to respond to a skid condition which requires alarge dump of pressure while preventing the system from responding torepeated pressure dumps caused bygear oscillation.

It is still a further object of this invention to provide for preskidinformation processing and skid information processing in antiskidcontrol systems while preventing continued skid control system responseto information generated in the system due togear oscillation occurringat the natural frequency of oscillation of the gear.

Other objects of this invention will become apparent from the followingdescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a block diagram of antiskid apparatus e the principles of theinvention;

FIG. 2 is a schematic diagram of the stabilizing circuit portion of theantiskid apparatus of FIG. I; I

FIG. 3 is a graph illustrating the relationships alonga time axisbetween the output of the valve driver and the status of the DC switch;

FIG. 4 is a block diagram of a further type of antiskid apparatus whichembodies the principles of the invention; and

FIG. 5 is a schematic diagram of a stabilizing circuit of the type shownin FIG. 2 connected in a portion of a circuit of the apparatus shown inFIG. 4.

In accordance with the invention, a timing means is actuated by removalof a skid-control signal represented by a valve dump voltage. Theactuation of the timing means further actuates a switch which does notpermit another dump voltage to be applied to the valve during the timingperiod of the timing means. The timing period of the timing means ispredetermined by choice of circuit constants of the timing circuit andis set after a determination has been made of the natural frequency ofthe landing gear structure. When the predetermined time period is set toexceedthe natural frequency of the landing gear then the switch will notallow a dump voltage within the natural period of the gear to pass tothe valve and dump pressure.

Referring now to FIG. lof the drawings, there is depicted in blockdiagram form an antiskid system illustrative of one class of systemsincorporating therein a stabilizing arrangement in accordance with theteachings of thepresent invention. Exembodying plary of this class ofsystems is that system described in 11.8.

Pat. No. 3,245,727 to Anderson et al. This class of systems developswheelspeed type information from wheel transducers. In FIG. 1 of thisapplication, wheelspeed type information which comprises theinputinformation to the antiskid .system is represented by block 10 whichincludes the wheel transducer, frequency sensing and wave shapingfunctions performed as in the control'diagram FIG. 2 of theabovereferred-to Anderson etfal. patent. The wheelspeed information thusdeveloped is transmitted to the skid detector portion of the systemrepresented by block 12 in FIG. I of this application. The skid detector12 includes circuitry as represented in FIG. 2 of Anderson et al. forintegrating wheelspeed for further providing the proper type of signalrelated to rate of change of wheelspeed for modulator utilization andalso provides for actual skid detection and consequent skid or dumpsignal generation. The dump signal is provided as one of the inputs tothe DC Amp. of. FIG. 2 of the above-identified patent while the otherinput to the DC Amp. comes from the modulator which includes the SlipRate Amp. and Slip Rate Threshold, the Valve Driverof FIG. 1 of thisapplication functioning to provide sufficientamplification of modulator30 and dump voltage 14 output signal levels to drive the valve 18 in thesame manner that the DC Amp. of FIG. 2 of the aboveidentified Andersonet al. patent drives its Valve. This class of skid-control systems hasthe advantage of controlling pressure release based on preskidinformation as well as controlling pressure based on actual skidinformation detected and processed. One-shot MV (multivibrator) 28 andDC switch 26 incorporated into the system of FIG. 1 in the particularmanner shown permit the system to respond to preskid information andgenerate a control signal by means including the modulator 30. Theparticular arrangement of one-shot multivibrator 28 and DC switch 26also permits skid information to generate a second form of controlsignal in the form of a dump voltage without permitting the systemperformance to deteriorate by permitting a second dump voltage to begenerated and again actuate brake release in response to a gearvibration at the natural frequency of the gear. The advantages of suchstabilization of the antiskid system are readily apparent since highgain and sensitivity to preskid information can be maintained while alsopermitting immediate dump voltage control of the valve to release brakepressure when a skid condition is indicated. A succession of largepressure dumps is not permitted when due to natural frequency vibrationof the landing gear.

In FIG. 1 it can be seen that the dump voltage (after amplification bythe valve driver) is sampled by coupling of the input terminal 32 of thestabilizing circuit arrangement in the output circuit path of the valvedriver. The valve dump voltage is thus seen as furnishing the inputsignal to the stabilizing circuit arrangement while the output terminal34 of the stabilizing circuit arrangement is coupled back to the dumpvoltage output circuit path in a manner that inhibits the subsequenttransmission of a second dump voltage to the valve driver 16 during thetime period of natural frequency of vibration of the landing gear as isfurther explained hereinafter in more detail. While blocks l0, l2, 14;16, I8, 22, 24, and 30 of FIG. 1 of this application may includecircuitry corresponding to that found in patent to Anderson et al. asdiscussed previously,

these blocks should not be considered tobe limited to circuitry thereindisclosed and incorporated herein by reference but should be consideredas representative of circuitry of systems of the same class known tothose skilled in the art and modifications thereof which can bepracticed by those skilled in the art. The features of this inventionwhich contribute the better system performance and improvedcharacteristics are provided by the circuitry in blocks 26 and 28 shownin detail in FIG. 2 and organized in the system as shown in FIG. 1.

Proceeding now to FIG. 2, there is shown in block 28 a oneshotmultivibrator circuit which has an input terminal 32 which may beadapted as shown in FIG. 1 for receiving the valve dump voltage signalwhich is the valve voltage signal represented by block 14, as amplifiedby the valve driver of block 16 in FIG. 1. In itsnormal state,transistor Q2 is in the on" condition with current flowing in thecollector-emitter path of Q2. Transistor O1 is in the off" conditionwith no current conduction in the collector emitter path. With thecircuit component values listed below for this circuit, the voltageacross R4 is approximately volts. At this time the collector of O2 is atabout 10.5 volts. The collector of O1 is at 16 volts. The base of O2 isapproximately 10.5 volts. When skid information is detected andprocessed by the system of FIG. 1 in the manner understood by thoseskilled in the art, a valve dump voltage is provided at the output ofthe valve driver 16 of FIG. 1 which energizes valve 18 to dump pressureand release brake 22, providing a release of braking effort on wheel 24in the manner understood by those skilled in the art. This valve dumpvoltage represented herein in the graph of FIG. 3 and which is providedat input terminal 32 of FIG. 2 forces the base of transistor 02 negativewhen the valve dump voltage is in the form of a negative going signal.The valve dump voltage is a negative going signal when it is dropping inamplitude towards 0 volts at the time it is being turned off as canreadily be seen when specific reference is made to the valve driveroutput voltage curve of FIG. 3 at time t When this negative going valvedriver output voltage which is at this time the amplified valve dumpvoltage is impressed on the base of transistor 02 by way of inputterminal 32 through input coupling network R9 and C2, then 02 stopsconducting and the voltage across R4 decreases. Transistor Q1 turns on(the collector emitter path of 01 becomes conductive) causing thevoltage on the collector ofQl to drop to a value of approximately 10volts which voltage is impressed on the base of transistor 02 by thecircuit path through capacitor C1 and which voltage remains until'thecurrent passing through R3 raises the voltage at the base of O2 to againturn on transistor 02. The charging time of timing capacitor C1 isdetermined by the component values of capacitor C1 and resistor R1. Ithas been found in practicing the present inventionthat in one embodimentthereof where the natural frequency of the landing gear was 10 cyclesper second which otherwise expressed is a time period. of 0.l0 seconds,a.predetermined charging time for capacitor C1 of milliseconds wasdesirable and was achieved by selecting a capacitor C1 having a value of4.4 microfarads and selecting a variable resistor R3 capable ofproviding a resistance value of 150,000 ohms. The time period for chargeof capacitor C1 equals the product of R3 and Cl. 7 .The resistor R3 maybe a variable type resistor as shown in FIG. 2 which permits the timeperiod selected to be varied above and below 130 milliseconds ifdesired. When transistor 02 is off as described above during thecharging period of capacitor C1, the collector voltage of transistor 02is l6volts which is a high enough value of voltage to cause the voltageestablished at the base of transistor Q3 through the circuit pathincluding diode CR2 and Zener diode VR2 and resistor R8 to turn ontransistor 03, thus closing theDC switch 26 by providing currentconduction from terminal'34 through the collector and emitter oftransistor 03 to ground. The voltage at the collector of transistor Q3and the terminal 34 connected thereto which drops when transistor O3 isturned on may be utilized to hold open the circuit path between thedumpvoltage represented by block 14 of FIG. 1 and the valve 18 at somepoint along this circuit path so that the valve 18 cannot be energized.In FIG. 5, transistor 03 is shown with collector coupled to outputterminal 34 which is in turn coupled to the base of transistor T3. Thecollector voltage of transistor 03 is low when transistor 03 isconducting or in the turned on con,- dition', and this collector voltagewhich is applied to the base of transistor Q3 prevents 03 from turningon and providing a circuit path through transistor Q3 for dump voltagecontrol of the means for releasing braking effort which includes thevalve. It is apparent from the foregoing how the switching means ofblock 26 may be utilized to interrupt the circuit path between a dumpvoltage control signal generated by skid information and the valveservopilot.

. Since the input terminal 32 of the stabilizing circuit is connected inthe dump voltage circuit path between the output of the valve driver 16and the input of valve 18 as seen in FIG. 1, the input terminal 32receives both modulator signal voltage signals which constitute firstcontrol signals capable of modu-' lating brake pressure and also dumpvoltage signals which constitute second control signals for rapidlyrelieving or dumping brake pressure. The one shot multivibrator 28 mustbe placed in its set state for a predetermined time period to cause DCswitch 26 to open the dump voltage to valve path for this predeterminedtime period but the one shot multivibrator comprising the timing means28 must respond only to a dump voltage control signal and not to amodulator signal voltage since it is desired to detect the occurrenceofa dump voltage and prevent the control effect of a second dump voltagewithout detection of and/or interference with the processing by theantiskid control system of modulator signal voltages. The circuitcomponent values selected for input network resistor R9 and inputnetwork capacitor C2 are determinative of the amplitude of triggersignal which will set the one shot multivibrator 28. In a typical systemutilizing the circuit of FIG. 2, the resistance R9 was selected to be5,000 ohms and the capacitance value of C2 selected was 2.0 microfarads.The effect of valve dump voltages and modulator voltages upon the.stabilizing circuit of FIG. 2 and the antiskid system of FIG. 1 which itstabilizes will be better understood with reference to FIG. 3; In FIG.3, the upper graph shows at time t= t that the output of the valvedriver is 0 volts which indicates to those skilled in the art that thewheelspeed detection arrangement of block 10 in FIG. 1 is nottransmitting the type of wheelspeed v information which requires thesystem through skid detector 12, modulator 30 and dump, voltage signaltranslating means 14 to modulate or rapidly dump brake pressure. At timet; it would also be recognized that skid information has been detectedand processed by the system to provide a valve dump voltage. This valvedump voltage causes release of brake pressure since it is applied as acontrol signal to the servopilot valve to bypass fluid under pressurearound the brake cylinder. From the graph it can be seen that this valvedump voltage rises to the value V,, which in practice of the circuitembodiment of FIG. 2 is about 8 or 9 volts. This rise of valve dumpvoltage at time t, or its steady value at time t;, does not set the oneshot multivibrator 28 since the positive change of voltage whichoccurred at time t, is a positive going signal not capable of turningoff transistor Q2 when applied to the base thereof and capacitor C2blocks the steady potential V occurring at time t t; from being appliedto the base of transistor 02. At time t t however, a negative goingvoltage is presented at terminal 32 and this negative pulse issufficient when applied through resistor R9 and capacitor C2 to triggertransistor Q2 into an off condition, causing the one shot at the base oftransistor 02 which turns off transistor 02.

Since valve dump voltage V 8 volts minimum and V 6 volts maximum, then VV 2 volts, and negative going differences of 2 volts or more will bepermitted to trigger multivibrator 28, and the desired mode of operationis achieved. The resistance value of R9 therefore does not permit smallnegative going difference voltages due to fluctuation in modulationvoltage to set the one shot multivibrator circuit 28 but permits only anegative going pulse of the type which only appears at the trailing edgeof the valve dump voltage and which is a larger difference voltage toset the multivibrator circuit. The resistance value of resistor R9, whenproperly selected as explained in the preceding, provides the means forachieving the important function of rejecting modulation information andpreventing its interference with the desired mode of operation of theantiskid control system circuit of the present invention by undesiredand untimely triggering of the multivibrator circuit. The negative goingvoltage difference represented by the trailing edge of the valve dumpvoltage at time t. sets the multivibrator circuit 28 causing DC switch26 to close by forming a closed circuit path from terminal 34 throughswitching transistor 03 to ground. The voltage at output terminal 34caused by the decreased collector voltage of 03 may be utilized as aswitching voltage to inhibit the flow of a valve dump voltage toenergize the servopilot valve causing the release of pressure andconsequent braking effort at valve 18 by interrupting the circuit pathat some point intermediate the dump voltage generated andthe servopilotvalve. While the first occurrence of valve dump voltage energizes theservopilot valve. the switch 26 is closed by this first occurrence ofvalve dump voltage opening the above-mentioned circuit path and thecircuit path is held open by the switch for the predetermined timeinterval from time t, to time t thus preventing any subsequent valvedump voltage signal generated from being transmitted along the circuitpath and energizing the servopilot'valve.

The following circuit component values were used successfully in theoperation of the stabilizing circuit embodiment of FIG. 2:

Rl=3 thousand ohms. R2=3 thousand ohms. R3= 150 thousand ohms variable.R4=5 thousand ohms. R9=5 thousand ohms.

C1=4.4 microfarads.

C2=2 microfarads. VR1=Zener diode, voltage drop= 10 volts. VR2=Zeuerdiode, voltage drop: 12 volts. Q1, Q2, Q3=Transistors, type 2N1711.

The component values given above are dependent upon the voltageoperating levels and system requirements but are easily computed bythose skilled in the art and should therefore not be considered limitingbut merely illustrative of one embodiment of the invention. Further,those portions of the system delineated in block diagram form in FIG. 1as pointed out previously which represent corresponding portions of theAnderson et al. system should not be considered limited to the circuitryof Anderson et al. performing the corresponding functions but theAnderson circuitry should be considered merely illustrative of oneexample of the circuitry which-may be utilized to perfonn thesefunctions.

Turning now to FIG. 4 of the drawings, thereis shown in block diagramform another system incorporating the teachings of the presentinvention. The system of FIG. 4 is illustrative of a further class ofsystems which can utilize the features of the present invention. Thefirst class of systems as represented by the system of FIG. 1 whichutilized the present inventive concepts, developed preskid informationand provided a control signal output from block 20 in FIG. 1 whichcaused the remaining portion of the system to modulate brake pressure inresponse to the preskid information processed. It can be noted bycomparison of the block diagrams of FIG. 1 and FIG. 4 that this furtherclass of systems represented in FIG. 4 does not include the modulator 32but only uses a dump voltage output from block 14 for control purposesin relieving brake pressure. It can be remembered from the abovediscussion in connection with the description of FIG. 1 that a dumpvoltage control signal is provided upon occurrence of an actual skid.The system of FIG. 4 can be seen therefore as not responsive to preskidinformation since not incorporating a modulator but being responsive toand providing a reduction in braking effort upon actual slipping of thewheel. US. Pat. No. 3,245,213 to Thompson et al. is illustrative of thisfurther class of systems. FIG. 4 represents this Thompson et al. typesystem in block diagram form modified in accordance with the teachingsof the present invention. The system of Thompson et al. is incorporatedherein by reference, with FIG. 5 of this application showingspecifically the block L of FIG. 2 portion of the Thompson et al. systemas modified to incorporate the stabilizing circuit in accordance withthe teachings of the present invention. The same legends of FIG. 2 ofthis application have been retained to identify the correspondingcircuit components in the FIG. 5 circuit of this application. Theremaining legends of FIG. 5 in this application are the same legendsused in FIG. 2 of the Thompson et al. patent and are retained herein toidentify lead lines which provide the external connections from block Lto the remaining portion of the Thompson et al. system. Input terminal32 is connected to line 22 to sense the negative going difference involtage provided by the trailing edge of a dump signal in the samemanneras heretofore described with reference to FIGS. 1, 2, and 3.Output terminal 34, as mentioned earlier in connection with thedescription of FIG. 2, provides a conductive path to ground (the DCswitch closes) for the predetennined time period that it is not desiredto pass a subsequent valve dump voltage. The output terminal 34 sincecoupled to the base of transistor T3 lowers the potential on the base oftransistor T3 when switching transistor 03 is closed and preventstransistor T3 from turning on (becoming conductive) and the path throughtransistor T3 is open therebyinhibiting the passage of a dump voltagetherethrough to rapidly dump brake pressure. After the predeterminedtime period has elapsed then the DC switch opens since switchingtransistor 03 stops conducting and the collector voltage of transistor03 as also applied to the base of transistor T3 rises permittingtransistor T3 to again become conductive and pass a dump signal torapidly release brake pressure upon the receipt by the system of furtherskid infor- Ination.

A previous approach to the solution of the problem of gear oscillationinterference with antiskid system performance may be illustrated byreference to US. Pat. No. 3,017,145 to Yarber. ln the Yarber patent, anarrangement is disclosed which delays response to skid signals by theutilization of time delay circuit means which thereby seeks to avoidgear oscillation by phase change of the skid correction signal. Thistype of solution to the problem of gear oscillation avoidance changessystem response thereby affecting system performance. The featuresdescribed in embodiments of the present invention do not affect the skidcontrolling signals and thus the response and consequent performance ofthe antiskid systems until just after a skid has occurred and then onlya frequency clamp is utilizedto prevent a second skid or dump signalfrom controlling brake pressure dump of that particular wheel on theexcited landing gear.

Since certain changes may be made in the above-described apparatus andcircuits and different embodiments of the invention may be made withoutdeparting from the spirit and scope thereof, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

We claim:

1. In an antiskid control system for a vehicle wheel brake, thecombination of:

means for generating a first control signal capable of modulating brakepressure in response to wheelspeed derived preskid information,

means for generating a second control signal capable of relieving brakepressure in response to wheelspeed derived skid information; and

means acting in response to said second control signal for preventingthe reapplication of said second control signal within a predeterminedtime period while permitting said first control signal to modulate brakepressure during said predetermined time period.

2. In an antiskid control system for a vehicle wheel brake, thecombination of:

skid detector means for generating a first control signal when a skiddevelops at the wheel from overly applied brake pressure;

first means including valve means actuatable to release the brake inresponse to said first signal;

second means responsive to said first control signal for producing asecond control signal having a predetermined time period; and

switching means responsive to said second control signal for preventingreapplication of said first control signal thereby causing release ofsaid input terminal, within said predetermined time period. period of 3.The combination defined in claim 2 in which said second means comprisesmonostable multivibrator circuit means having a trigger input terminal,an output terminal, and time interval determining network means forsetting the predetermined time period ofsaid second control signalgenerated at said output terminal when said first control signal iscoupled to said trigger input terminal.

4. The combination defined in claim 2 in which said switching meanscomprises transistor switching circuit having an input terminal coupledto the output terminal of said monostable multivibrator circuit means,and an output ter- .ninal arranged to switch off said first controlsignal thereby preventing energization of said first means during saidpredetermined time period.

5. A brake-control system for controlling application of braking effortto a wheel of a vehicle, said system comprising, in combination with thewheel and associated brake:

antiskid control signal generating means for generating a first controlsignal;

means responsive to said first control signal to relieve braking effortapplied to said-wheel by the associated brake;

means responsive to said first control signal for generating a secondcontrol signal having a predetermined time period; and

mined time period comprises monostable multivibrator circuit 4 meanshaving a trigger input terminal coupled to receive said first controlsignal and actuable to provide said second control 1 signal when saidfirst control signal'is representative of max;:

imum release of braking effort'f'said multivibrator circuit means havingan output terminal for providing said second control signal, said meansactuable by said second coiitrol signal comprising a switching circuitcontrolled by said second control signal and acting to inhibit controlof braking effort when said second control signal is produced by saidmultivibrator circuit.

7. A brake-control system for controlling application of braking effortto a wheel of a vehicle having a plurality of independently-rotatablebrake-equipped wheels, said system comprising, in combination with thewheel and associated brake:

means for developing from wheelspeed variation an antiskid controlsignal;

means coupled to receive said antiskid control signal and responsive tosaid control signal to relieve braking effort applied to the wheel bythe associated brake over a range which includes a maximum value forrelieving brake effort; and

means coupled to receive a said antiskid control signal and includingtiming means initiated in action by said antiskid control signal andacting to'prevent reapplication of said maximum value of release ofbrake effort after the first occurrence of said maximum release of brakeeffort. 8. In an antiskid braking system, the combination of: arotatablewheel;

p10 means to supply hydraulic braking pressure to the wheel;

valve means to adjustably restrict the hydraulic pressure actuallyapplied to the wheel;

means to represent the wheel rotation as an electrical signal;

and skid detector means coupled to said means representing wheelrotation so as to control the braking pressure to the wheel by supplyinga control signal through circuit means to the valve means, said circuitmeans inhibiting the transmission of said control signal through saidcircuit means to the valve means when said control signal is repetitiveof a predetermined value during a predetermined time interval. 9. Thecombination defined in claim 8, and in which said circuit means includestime actuatable switching means coupled in circuit path with saidcontrol signal.

10. An antiskid control system stabilizing circuit comprismg: t

a monostable multivibrator circuit having an input terminal, an outputterminal, and a time determining network for controlling the timeduration of an output pulse generated at said output terminal inresponse to an input signal applied to said input terminal, said inputterminal being coupled to receive an antiskid control signal forcontrolling application of braking effort to a wheel; and

switching means coupled to said output terminal and controlled by saidoutput pulse, said switching means arranged to inhibit the reapplicationof said antiskid control signal in said antiskid control system duringthe time period of said output pulse thereby preventing control ofbraking effort during said time period.

11. An antiskid control system stabilizing circuit according 7 to claim10 wherein said time determining network includes variable resistancemeans for varying the duration of said output pulse.

12. An antiskid control system stabilizing circuit according to claim 10wherein said input terminal includes means cou- 75 pled in seriestherewith for preventing said output pulse from being generated at saidoutput terminal when said input signal is below a predetermined negativedifference voltage value.

13. An antiskid control system stabilizing circuit according to claim 10wherein said-time determining network has a time constant exceeding thenatural period of the landing gear structure. r

14. An antiskid control system stabilizing circuit according to claim 13wherein said time constant is about 130 milliseconds.

15. ln a control system for a brake of an aircraft wheel which iscarriedby longitudinally flexible landing gear structure having apredetermined natural period of oscillation, the combination of:

means driven in accordance with rotation of the braked wheel fordeveloping an alternating current voltage substantially proportional tothe speed of the wheel;

3 skid detector means responsive to said alternating current voltage andacting to produce a skidcontrol signal; means to relieve the brake inresponse to said skid control signal; and frequency clamping means toprevent reapplication of said skid control signal to said means forrelieving the brake within the natural period of oscillation of thelanding gear structure.

16. The combination defined in claim 15 wherein said frequency clampingmeans includes a timing element comprising a multivibrator circuit, saidclamping means further including switching means coupled to saidmultivibrator circuit and controlled thereby, said switching meansactuable to ar rest reapplication of said skid control signal within thenatural period of oscillation of the landing gear structure therebypreventing a second skid signal from the excited gear.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION December 8, 19703,545,819 Dated William Charles Gaffney et al Patent No.

Inventor(s) It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 7 lines 50 and 51 "thereby caus ing release of sai inputterminal, of" should read M said predetermined time period.

Signed and sealed this 25th day of May 1971 (SEAL) Attest:

EDWARD M. FLETCHER ,JR. Attesting Officer WILLIAM E SCHUYLER, JR.Commissioner of Patents within said predetermined time period. periodthereby causing release of said brake withi:

USCOMM-DC FORM P0-1050 (10-69)

